The present invention relates to an annular gasket for use in the moulding of ophthalmic or optical lenses of organic material.
As is known, such lenses are normally moulded between two mould shells which are spaced from one another by an annular gasket which forms a distance-piece and ensures the tightness of the mould cavity defined between the mould shells.
Currently, in order to keep the mould shells in contact with the annular gasket separating them an elastic clamp is used which bears conjointly upon both of these mould shells from both sides of the annular gasket.
As is also known, the most commonly used organic materials for moulding ophthalmic or optical lenses require heating on order to set, this setting corresponding to a polymerisation, and in the course of this setting a not inconsiderable shrinkage occurs in the material.
Naturally, the mould shells must be able to follow such a shrinkage.
Initially, the mould shells follow this shrinkage because of their own elasticity under the joint action of the moulded material and the elastic clamp. However, the gasket interposed between the mould shells opposes any movement by the shells which would permit them to follow a more accentuated shrinkage of the material. Accordingly, this gasket must subsequently be removed and be replaced by a viscous product, in order to ensure that the polymerisation in progress will proceed under exclusion of air.
With such a process, therefore, not only must an annular gasket and an elastic clamp be used initially but also both the gasket and the clamp must subsequently be removed.
Such a process is complicated and, therefore, expensive. In addition, it involves the division of the heat treatment required for the polymerisation into two phases, namely a prepolymerisation phase at moderate temperature, with the gasket and clamp, and then, after removal of these items, a definite polymerisation phase at increased temperature. The inevitable result is that certain lenses produced in this manner have faults leading to their rejection.
However, in U.S. Pat. No. 3.555.610 it has been proposed to use an annular gasket which allows this process to be simplified and avoids any division of the heat treatment. This gasket has at its internal periphery an annular ridge which is formed at the end of an axially elastically deformable annular flange which enables it to cooperate tightly with one of the mould shells.
The capacity for axial elastic deformation of the annular flange makes it possible for the mould shells to approach one another after the material to be moulded has been put into place. Upon the release of the mould shells the annular flange of the gasket tends to return them to their initial configuration and thus a suction effect is created between the shells. This suction effect is sufficient to ensure, with complete reliability, that each of these mould shells is suitably kept in contact with the gasket. It is not then necessary to employ any type of elastic clamp to keep the shells in contact with the gasket. Furthermore, the gasket can be left in place during the final polymerisation stage of the moulded material, the elasticity of the annular flange permitting the gasket to follow the relative movement which is necessary so that the mould shells can follow the shrinkage of this material completely during the course of its polymerisation.
However, in the above mentioned U.S. Patent, the proposed annular flange only extends axially and only projects from the corresponding transverse face of the gasket. As a result, its elastic deformation capacity is essentially due to its capability of being crushed.
Such capabilities for crushing are in reality hard to control and, as previously, result in manufacturing problems.
Besides, in the above mentioned U.S. Patent, no particular arrangement has been proposed for controlling the movement of closure exercised between the mould shells while they are being placed in position.
This can result in considerable differences in the thickness of the lenses obtained, from one production to the next.
Although such differences in thickness do not constitute a major detect in half-finished lenses which undergo a final machining process providing them with their definite configuration, this is not the same for finished correction lenses, or afocal lenses, and more particularly for sunglasses, since basically no machining process is applied to these after they have been moulded.
In general terms, it is an object of the present invention to reduce these disadvantages.